An EC probe is normally manufactured using an electric conductor and winding it into a coil. The conductor has electric resistance, and this conductor resistance becomes an unwanted part of the measurement. This is because part of the conductor resistance can be often confused with the EC measurements being made. A typical instrument needs to be calibrated by the user at the start of operation to remove as much as possible the effects of the probe conductor resistance. However, the task of eliminating the effect of EC probe conductor resistance becomes challenging because the resistance is a function of temperature. As the operational temperature changes, the probe conductor resistance changes, introducing changes in measurement results unknown to operator and therefore un-accounted for during the calibration procedures. As a result, the EC measurement that the instrument performs becomes less accurate.
A typical EC instrument applies an alternating current in the range of 1 KHz to 12 MHz to the probe coil. By measuring the probe voltage and current components in phase and 90 deg to the voltage phase, the resistance and the inductance of the probe can be calculated, if there are no errors caused by factors such as temperature change.
In an existing practice, to make a typical measurement or inspection of a test sample, for example, an aluminum plate, the operator would place the probe against the plate. The EC instrument will generate an alternating current and then measure the inductance and the resistance in the EC probe (hereinafter as the measured inductance and the measured resistance). The eddy current induced into the test piece, bucks the field of the probe, and thus the field is squeezed into a smaller space. A smaller magnetic field requires less energy to generate. Less energy being used implies that the instrument will measure less inductance in the EC probe.
The EC instrument measures the energy lost during the measurement. The test sample in which the eddy current flows is not a perfect electrical conductor and therefore responsible for a portion of the energy loss in this process. The EC instrument's measured resistance actually represents the probe conductor resistance, plus the resistance to the eddy current in the test sample. The EC instrument measurement of resistance can not distinguish between the energy lost by the test sample resistance, and the energy lost by the probe conductor resistance. In existing practice, the change in the probe conductor resistance is not accounted for, and therefore the change in energy lost in the probe conductor resistance is also not accounted for.
In order to overcome the inaccuracy caused by the above described factors, and to measure the eddy current energy loss accurately, the instantaneous probe conductor resistance must be known. It would be preferable if the instantaneous probe conductor resistance or the amount of change from a baseline can be presented to the instrument display and/or accounted for in an algorithm to calculate the energy lost by the eddy current in the test sample.
Existing practice has been seen in U.S. Pat. Nos. 4,893,079 and 6,541,963 in efforts trying to compensate EC inspection drifts caused by temperature change.
U.S. Pat. No. 4,893,079 describes a method for measuring physical characteristics of an electrically conductive material by the use of eddy current techniques and compensating measurement errors caused by changes in temperature. It includes a switching arrangement connected between primary and reference coils of an eddy current probe which allows the probe to be selectively connected between an eddy current output oscilloscope and a digital ohm-meter for measuring the resistances of the primary and reference coils. The changes in resistance due to temperature effects are taken into account while determining the eddy current measurement. However, the need for the extra reference coil in this patent increases the size and the cost of the probe and limits the range of the inspection that the probe can access to.
U.S. Pat. No. 6,541,963 describes an EC transducer capable of enhancing the measurement accuracy by compensating for temperature errors, increasing the resolution and noise immunity. The primary detector in this differential EC transducer incorporates two similar search coils and an additional coil, which presents a similar drawback as seen in U.S. Pat. No. 4,893,079.
The invention described here employs neither the additional coils nor the modifications made to the EC transducer per U.S. Pat. No. 6,541,963. It instead employs a single coil probe and addresses the problem on the EC instrument side by using a low frequency signal generator.
Thus it would be advantageous to provide an EC system with a probe that can compensate the affect of temperature change without the need for an additional coil or an extra bridge circuit as seen in existing efforts.